We hypothesize that the lens epithelium is able to sense and respond to changes in the entire cellular structure and adjust Na,K-ATPase activity accordingly. To function normally, maintain ion homeostasis, establish a membrane potential and regulate water content, all cells require active Na-K transport provided by Na,K-ATPase. The lens, however, is made up almost entirely of fiber cells that have little or no Na,K-ATPase activity. Lens ion and water homeostasis is made possible by a relatively few cells at the periphery of epithelium monolayer that somehow manage to match Na,K-ATPase activity to the needs of the entire cellular structure. Our studies in the last funding period indicate TRPV4 channels are able to trigger an increase of Na,K-ATPase activity, effectively making the Na,K-ATPase mechanosensitive. This enables the epithelium to detect and respond to swelling caused by ion imbalance in distant regions of the lens. Aim 1 is a study of the sensor mechanism. We propose experiments to explain how mechanosensitive TRPV4 ion channels detect swelling and cause the epithelium to emit a purinergic agonist, ATP. The ATP causes a response that stimulates Na,K-ATPase. Aim 2 is a study of the response mechanism. We propose experiments to explain how receptors, protein kinases and the regulatory protein Fxyd6 interact to increase Na,K- ATPase activity. Lens transparency depends on ion and water homeostasis. These lens studies will help explain how homeostasis is achieved. The significance to human well- being is the occurrence of cataract when homeostasis fails.